1. Field of the Invention:
The present invention relates to a tire air pressure warning system for detecting lowering in pressure of a tire and issuing a warning about the pressure lowering.
2. Description of the Related Art:
In order to measure an air pressure inside a tire (hereinafter, simply called tire air pressure or air pressure) of a vehicle such as an automobile, there are hitherto provided an apparatus including a tire air pressure gauge disposed at a wheel and a radio transmitter for transmitting an air pressure signal including a result of measurement carried out by the tire air pressure gauge, and a receiver for receiving the air pressure signal (disclosed in, for example, Japanese Patent Application KOKAI (Laid-Open) Publication No. HEI 11-20427, HEI 11-321254, Japanese Patent Publication No. 2716335).
Such an apparatus arranges the tire air pressure gauge and the radio transmitter inside a wheel, and measured tire air pressure data, which is measured by the tire air pressure gauge, is transmitted in the form of a signal by the radio transmitter as required. On the other hand, the receiver is disposed on the bodywork portion of the vehicle and receives the measured air pressure data transmitted by the radio transmitter so that the tire air pressure can be monitored.
It is convenient for the above technique that the timing for measurement of a tire air pressure or for transmitting a radio signal can be controlled from the bodywork portion so that information of the tire air pressure such as a measured air pressure can be obtained as required. But, in order to control operation performed by a tire air pressure gauge and a radio transmitter placed inside a wheel from outside and other than the wheel, the apparatus however requires another receiver disposed inside the wheel for receiving a control signal. The narrow space inside the wheel (inside the tire) forces a receiver to be small in size and light in weight in order to be accommodated inside the wheel, resulting in increased costs and a complex manufacturing process. Cable connection between a wheel and a bodywork portion is not practically realized unless a slip ring or another part is not installed at the axis of the wheel.
For this reason, in a general tire air pressure measurement apparatus, a tire air pressure gauge and a radio transmitter, which are installed at a wheel, is predetermined to periodically operate at regular intervals without requiring control from the bodywork portion. For example, the tire air pressure gauge measures a tire air pressure once an hour when the vehicle is parked and once a minute when the vehicle is traveling, and the radio transmitter transmits a radio signal including the measured air pressure data to the surroundings.
Further, the apparatus generally determines a threshold value with hysteresis for judgment as to whether or not a warning is required. In other words, a threshold value (a warning onset threshold value) at which a warning is started is set different in value from a threshold value (a warning termination threshold value) at which the warning is terminated. Specifically, when a warning is not issued, a tire air pressure lower than the warning onset threshold value causes a warning to be issued; and while a warning is issued, a tire pressure higher than the warning onset threshold value does not cause the warning to be cancelled but a tire pressure higher than the warning termination threshold value, which is higher than the warning onset threshold value, causes the current warning to be cancelled.
If a warning against a tire air pressure is judged on the basis of a single warning threshold value, a result of judgment made on a tire air pressure around the threshold value is not stable. Conversely, a hysteresis area between the warning onset threshold value and the warning termination threshold value can obtain a stable judgment result.
A warning issued in an apparatus for measuring a tire air pressure described above is exemplified by making a noise with the horn, by lighting an external lighting device such as the headlamps and/or the flashers, by lighting an interior lighting device such as the interior lamp, and/or by using a buzzer and an equipped warning. In any case, a preferable warning is issued in accordance with the state of the vehicle. Namely, when the driver is inside the vehicle while the vehicle is traveling or is stopped, a preferable warning is issued inside the vehicle using the above buzzer or lamp exemplified by the interior lamp; when the driver is not on board the vehicle while the vehicle is stopped, a preferable warning is issued outside the vehicle using the horn, the headlamp, the flashers and/or other device.
For vehicles that also have a so-called keyless entry system in addition to an apparatus for measuring tire air pressure, another technique proposes that signals for the system and the apparatus utilize the same radio frequency and share the same receiver so that a distinct warning is issued inside or outside the vehicle in accordance with an activating state of the door locking mechanism (disclosed in, for example, the above-mentioned Japanese Patent Publication No. 2716335 hereinafter also called patent reference #3).
In patent reference #3, while a vehicle is traveling or if the vehicle does not receive neither door lock signal nor door unlocking signal during stopping, detection of an improper tire air pressure issues a warning inside the vehicle; and while a vehicle is stopped after the doors are unlocked in response to receipt of a door-unlock signal, opening of the doors without receiving a door-unlock signal or detection of an improper tire air pressure causes a warning to be issued outside the vehicle.
Although a tire air pressure gauge does not detect an improper tire air pressure at the time when a vehicle is stopped for parking, the tire air pressure measuring apparatus may detect an improper tire pressure (a lowered tire air pressure) which is caused by variation in a tire pressure due to a nail piercing the tire or other reason while the vehicle is parked, i.e., while a switch (the main power switch exemplified by the ignition switch) is off, the doors are locked and the driver is absent from the inside of the vehicle.
In the case of such a variation in tire air pressure, the tire air pressure gauge measures tire air pressure and the radio transmitter issues a radio signal including information of the measured pressure data even when the IG switch is in the off state because the conventional tire air pressure measuring apparatus is set to periodically operate at regular intervals irrespective of an on/off state of the IG switch. Upon receipt of the measured air pressure data included in the issued radio signal by the receiver, a judgment is made for the measured air pressure and, if the tire pressure is lowered to a lower level than the warning onset threshold value, the judgment concludes that a warning should be issued although a warning is not actually issued. After that, when the driver unlocks the doors, boards the vehicle and turns the IG switch on, a warning is issued.
As mentioned above, the conventional tire air pressure measuring apparatus does not notify a driver of an improper tire air pressure (a lowered tire air pressure) until the driver boards the vehicle and turns the IG switch on. Therefore, the driver has to get out the vehicle in order to check the tire with improper air pressure and carry out rectification (e.g., refill the tire with air or replacing the tire with a spare tire), being loaded with increasing labor.
In a technique disclosed in patent reference #3, when a vehicle is parked and the driver is not on board the vehicle, the vehicle performs an operation in an alert mode for prevention of car theft. Thereby, when the tire air pressure gauge detects an improper tire air pressure (a lowered tire air pressure) while the vehicle is parked, the receiver subsequently receives a measured air pressure signal and the apparatus judges that a warning should be issued, a warning is automatically issued outside the vehicle.
In other words, since, in the technique disclosed in Patent reference #3 or the like, since the vehicle being parked also issues a warning for crime prevention, the vehicle issues a warning irrespective of the presence of the driver inside and around the vehicle even when the driver is not in proximity to the vehicle and is therefore not aware of the warning. For this reason, a variation in tire air pressure caused by a reason other than theft or mischief issues a warning indiscriminately and the warning may be a noise.